Interfacing, the word is used a lot today to describe plugging an electrical gadget into you computer.
But in crafting terms it is a product used for stiffening fabric.
There are different types depending on what finish you what on the item you are making. For example, a bag needs to be stiff, so you would use heavy interfacing. but for a collar of a blouse made for shear fabric only a lightweight is needed.
There are also different colours, so a dark fabric would have a dark interfacing.
There is also sew in and iron on.
And there are lots of specialist interfacing.
The best thing to do to navigate the variety and range is to look at the pattern you are creating, and buy what is suggested.
I also keep on hand medium weight interfacing as it is a good all-rounder for moth projects.